Cell differentiation is the central characteristic of tissue morphogenesis which initiates during embryogenesis, and continues to various degrees throughout the life of an organism in adult tissue repair and regeneration mechanisms. The degree of morphogenesis in adult tissue varies among different tissues and is related, among other things, to the degree of cell turnover in a given tissue.
The cellular and molecular events which govern the stimulus for differentiation of cells is an area of intensive research. In the medical and veterinary fields, it is anticipated that discovery of the factor or factors which control cell differentiation and tissue morphogenesis will advance significantly the ability to repair and regenerate diseased or damaged mammalian tissues and organs. Particularly useful areas for human and veterinary therapeutics include reconstructive surgery, the treatment of tissue degenerative diseases including, for example, arthritis, emphysema, osteoporosis, cardiomyopathy, cirrhosis, degenerative nerve diseases, inflammatory diseases, and cancer, and in the regeneration of tissues, organs and limbs. The terms “morphogenetic” and “morphogenic” are often used interchangeably.
A number of different factors have been isolated in recent years which appear to play a role in cell differentiation. Recently, a distinct subfamily of the “superfamily” of structurally related polypeptides referred to in the art as the “Transforming Growth Factor-beta (TGF-beta) superfamily of polypeptides” have been identified as true tissue morphogens.
The members of this distinct “subfamily” of true tissue morphogenic polypeptides share substantial amino acid sequence homology within their morphogenetically active C-terminal domains, including a conserved six or seven cysteine skeleton, and share the in vivo activity of inducing tissue-specific morphogenesis in a variety of organs and tissues. The polypeptides apparently contact and interact with progenitor cells e.g., by binding suitable cell surface molecules, predisposing or otherwise stimulating the cells to proliferate and differentiate in a morphogenetically permissive environment. Recent studies on cell surface receptor binding of various members of the TGF-beta polypeptide superfamily suggests that the peptides mediate their activity by interaction with two different receptors, referred to as Type I and Type II receptors. The Type I or Type II receptors are both serine/threonine kinases, and share similar structures: an intracellular domain that consists essentially of the kinase, a short, extended hydrophobic sequence sufficient to span the membrane one time, and an extracellular domain characterized by a high concentration of conserved cysteines.
These morphogenic polypeptides are capable of inducing the developmental cascade of cellular and molecular events that culminate in the formation of new organ-specific tissue, including any vascularization, connective tissue formation, and nerve innervation as required by the naturally occurring tissue. The polypeptides have been shown to induce morphogenesis of both bone cartilage and bone, as well as periodontal tissues, dentin, liver, and neural tissue, including retinal tissue.
These true tissue morphogenic polypeptides are recognized in the art as a distinct subfamily of polypeptides different from other members of the TGF-beta superfamily in that they share a high degree of sequence identity in the C-terminal domain and in that the true tissue morphogenic polypeptides are able to induce, on their own, the full cascade of events that result in formation of functional tissue rather than merely inducing formation of fibrotic (scar) tissue. Specifically, members of the family of morphogenic polypeptides are capable of all of the following in a morphogenetically permissive environment: stimulating cell proliferation and cell differentiation, and supporting the growth and maintenance of differentiated cells. The morphogenic polypeptides apparently also may act as endocrine, paracrine or autocrine factors.
As a result of their biological activities, significant effort has been directed toward the development of morphogen-based therapeutics for treating injured or diseased mammalian tissue, including, for example, therapeutic compositions for inducing regenerative healing of bone defects such as fractures, as well as therapeutic compositions for preserving or restoring healthy metabolic properties in diseased tissue, e.g., osteopenic bone tissue.